Double-sided polishing machines serve for polishing workpieces having coplanar surfaces, e.g. wafers. The plane parallelism is an essential quality criterion.
A double-sided polishing machine usually has two working discs which are rotationally driven by a shaft, preferably in opposite sense to each other. The shafts are co-axial. Each working disc includes a polishing disc and a carrier disc. Usually, the polishing disc has a steel disc which is firmly secured to the carrier disc. The facing surfaces of the polishing disc are covered by a polishing pad. The flat workpieces are accommodated by openings of so-called runner discs which can be rotationally driven by means of a tooth ring or a pin ring respectively. The workpieces move along a cycloidal path between the working disc or the polishing gap, respectively. The effective polishing surface of the upper and lower working disc thus is an annular surface.
Usually, a polishing medium is introduced into the polishing gap in form of a suspension. It is known to use the polishing medium for cooling purposes in that it flows on one side through passages between the carrier and the polishing disc and on the other side into the polishing gap through axial parallel bores in the polishing disc. It is further known to provide cooling passages between the carrier disc and the polishing disc (labyrinth) into which cooling medium is introduced e.g. water. The cooling medium is fed through axial passages in the driving shaft which are connected to an outer stationary cooling source through a rotary coupling.
Since the workpieces are moved in the polishing gap between upper and lower polishing disc the geometry (parallelism of the surfaces) is considerably determined by the geometry of the polishing discs i.e. through the difference in geometry between the upper and the lower polishing discs i.e. the polishing gap.
As already mentioned the temperature necessary for polishing performance can be controlled by a suitable cooling medium. At the beginning of the polishing step the temperature is for example 40° C. During the polishing process a considerable process temperature is generated. This deforms the polishing discs. As mentioned carrier disc and polishing disc are firmly interconnected and thus result in a different extension of both discs so that the working surface of the concerned polishing disc attains a convex shape. If the temperature in the polishing disc is reduced this results in a uniform polishing gap again.
The object of the invention is to provide a double-sided polishing machine wherein at any time an approximately uniform polishing gap is achieved throughout the radius of the polishing surface.